This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to improving temporal resolution in dynamic imaging where an imaged object includes both static material and dynamic material.
Magnetic resonance imaging (MRI) is a nondestructive method for the analysis of materials and represents a new approach to medical imaging. It is generally noninvasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited using magnetic fields which rotate at specific frequencies proportional to the local static magnetic field. The radio frequency signals resulting from the precession of excited spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin distribution of the body.
FIG. 1A is a perspective view, partially in section, illustrating coil apparatus in MR imaging system and FIGS. 1B-1D illustrate field gradients which can be produced in the apparatus of FIG. 1A. Briefly, the uniform static field B.sub.0 is generated by the magnet comprising the coil pair 10. A gradient field G(x) is generated by a complex gradient coil set which can be wound on the cylinder 12. An RF field B.sub.1 is generated by a saddle coil 14. A patient undergoing imaging would be positioned along the Z axis within in the saddle coil. In FIG. 1B an X gradient field is shown which is parallel to the static field B.sub.0 and varies linearly with distance along the X axis but does not vary with distance along the Y or Z axes. FIGS. 1C and 1D are similar representations of the Y gradient and Z gradient fields, respectively.
FIG. 2 is a functional block diagram of conventional imaging apparatus. A computer 20 was programmed to control the operation of the MRI apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22 and the RF coils for impressing an RF magnetic moment at the Larmor frequency is controlled by the transmitter 24 and the RF coils 26. After the selected nuclei have been flipped, the RF coils 26 are employed to detect the FID signal which is passed through the receiver 28 and then through digitizer 30 for processing by computer 20. For the dynamic imaging techniques of the present invention, physiological monitoring and triggering equipment (not shown) may be needed, as known by one skilled in the art.
Most MRI is performed using methods that require multiple experiments, or sequence repetitions, to obtain the desired spatial information. The most commonly used method, Spin Warp imaging, performs for example 128 or 256 sequences, each with a different area of the "phase encoding" lobe. The area of the phase encoding lobe is typically controlled by adjusting the amplitude of the waveform. The echo acquired with each phase encoding area corresponds to a line in the Fourier transform of the image (k-space), which line is acquired being determined by the area of the phase encoding lobe. The method of the present invention is applicable to many other imaging methods. It will be described with respect to Spin Warp imaging first. Examples of applicability to other methods will be described later.
For dynamic imaging, for example for forming images as a function of time in the cardiac cycle, one needs to acquire data with all the necessary phase encoding areas for each portion of the cardiac cycle. One technique for performing dynamic imaging is called segmented k-space scanning, and its operation is shown in FIG. 4. This technique uses a pulse sequence repetition time TR which is very short compared to the cardiac period, on the order of 10 ms. In the example of FIG. 4, in the first cardiac cycle of the scan, the measurements denoted by the open circles are acquired. The system collects data with the first 8 values of the phase encoding lobe; these will contribute to the first time frame of the dynamic image set. The system then acquires the first 8 phase encodes for frame 2, and so on throughout the cardiac cycle. The measurements made during the second cardiac cycle are shown in FIG. 4 as crosses. The system collects phase encodings 9 through 16 for the first time frame, then the same phase encodes for the next time frame, and so on. Thus, eight phase encodes are collected for each time frame during each cardiac cycle. Typically, the scan is on the order of 16 heart cycles long to enable the acquisition of all the needed data; five cycles are shown in FIG. 4. The data that contributes to the second time frame are enclosed in the box in FIG. 4. As can be appreciated, the temporal resolution of this image is 8TR.
The spacing of phase encoding amplitudes is selected so as to achieve the desired field of view (FOV) in the phase encoded direction. If it was known that the object was smaller than the FOV of the acquisition of FIG. 4, the acquisition of FIG. 5 could be used. This acquisition uses half the number of phase encoding amplitudes in each temporal frame but doubles their spacing. This produces the same spatial resolution as in FIG. 4 but over half the FOV. For example, as shown, in the first cycle phase encodings 1, 3, 5 and 7 (relative to those of FIG. 4) are collected for each frame. Because the amount of data is halved, the signal to noise ratio (SNR) is reduced by .sqroot.2. However, because the number of needed phase encodings is halved the number of phase encodings collected for each time frame in each cycle can be halved, and therefore the temporal resolution that can be achieved in the same total acquisition time is improved by a factor of 2, as shown by the narrowed box in FIG. 5. In this example, the odd numbered phase encodings were used. Substantially equivalent results would be obtained if the even numbered phase encodings had been used.
The present invention is directed to improving the temporal resolution of MRI data and images for an object that has known static material and known dynamic material. The article "Reduction of Field of View for Dynamic Imaging" by Hu and Parrish, published in Magnetic Resonance in Medicine, Vol. 31, pp. 691-694, 1994, describes a method for improving the temporal resolution in a dynamic study if the dynamic portion of the object is known to occupy no more than half the full FOV. The difference between any time frame and a first time frame is used to produce a difference image. The difference image is insensitive to the static portion of the object. A .sqroot.2 loss in SNR is expected due to the temporal resolution improvement if the dynamic region occupies half the full FOV. However, this method suffers an additional .sqroot.2 reduction in SNR due to the subtraction operation. In addition, the authors report sensitivity to slow signal variations which cause artifacts. The method of the present invention delivers higher SNR and is less artifact prone than this prior art alternative.